Target structure for a vidicon tube and methods of producing the same

ABSTRACT

A vidicon tube target structure wherein a n-conductive signal plate has a target surface thereof provided with a p-conductive layer and an array of fine grooves extend from this layer downwardly beyond the p-conductive layer into the signal plate to define an array of mesa-like p-conducitve land areas. The peripheral groove walls are coated with an insulator material while the land areas are coated with a conductive metal. The grooves and lands are configured to provide an increased amount of utilizable effective target surface area for a scanning electron beam.

United States Patent Veith 51 Dec. 26, 1972 [54] TARGET STRUCTURE FOR AVIDICON TUBE AND METHODS OF PRODUCING THE SAME [72] Inventor: WernerVelth, Munich, Germany [73] Assignee: Siemens Aktiengesellschnit, Berlinand Munich, Germany 221 Filed: Dec. 2, 1970 211 Appl. No.: 94,382

[30] Foreign Application Priority Data Dec. 3, 1969 Germany ..P 19 60705.7

[52] US. Cl....3l7/235 R, 317/235 NA, 317/235 AJ, 317/235 AK [51] Int.Cl. ..H0ll 15/00 [58] Field of Search 1317/235 N, 235 R, 235 A], 235 AK[56] References Cited UNITED STATES PATENTS 3,569,758 3/1971 l-ioriuchi..313/66 3,432,919 3/1969 Rosvold ..29/578 3,581,151 5/1971 Boyle....317/234 3,564,309 2/1971 Hart ..313/66 Primary Examiner-Martin H.Edlow Attorney-Hill, Sherman, Meroni, Gross & Simpson [5 7] 1 ABSTRACT Avidicon tube target structure wherein a n-conductive signal plate has atarget surface thereof provided with a p-conductive layer and an arrayof fine grooves extend from this layer downwardly beyond thep-conductive layer into the signal plate to define an array of mesa-likep-conducitve land areas. The peripheral groove walls are coated with aninsulator material while the land areas are coated with a conductivemetal. The grooves and lands are configured to provide an increasedamount of utilizable effective target surface area for a scanningelectron beam.

7 Claims, 4 Drawing Figures TARGET STRUCTURE FOR A VIDICON TUBE ANDMETHODS OF PRODUCING THE SAME BACKGROUND OF THE INVENTION 1. Field OfThe Invention The invention relates to a light sensitive storage targetstructure for television camera tubes, and more particularly to avidicon target structure.

2. Prior Art Vidicon target structures are known. For example, U. S.Pat. No. 3,403,284 describes a particular form of a vidicon targetstructure comprised of a Si-diode vidicon that has pn-junctions atcircular openings of a n-conductive signal plate. Such prior artstructures are formed by photo-screen techniques so that the resultantdiodes have pn-junctions that are somewhat spherical or curved. Theexposed target surface areas between such diodes (which define theutilizable target surface area) are coated with an insulator material ina manner so as to somewhat overlap peripheral edges of the diodes. Thistype of vidicon target structure has found some utility since it hasmechanical sturdiness, fair sensitivity and a relatively short decaytime associated with silicon, i.e., a low smearing effect.

Nevertheless, these prior art target structures have a number of seriousdrawbacks. For example, the insulator layer between the array ofindividual diodes carries a negative potential or load. This negativepotential produces such an interfering effect to the scanning process bya slow electron beam that the positive potential or load formed on thediodes by the incoming photons is extremely difficult to neutralize, andnormally is only partially neutralized. Further, a capacitance is formedat the (i.e., boundaries defining the circular openings of the Si-platei.e., the signal plate) and noticeably interferes with proper picturestorage.

An equalization layer has been proposed for alleviating suchinterference effect, which is exemplarily composed of an antiminoustrisulfide. However, use of an equalization layer materially impairs thequality and reliability of the resultant target structure. It isdifficult to obtain the correct conductivity within such an equalizationlayer to achieve the desired potential or load neutralization. Further,the equalization layer has its own photo effect and thus causes aninterfering smearing effect to be noted during operation. Additionally,the equalization layer materially limits the bake-out temperatures oftubes containing them and thereby reduces the useful life period of suchtubes.

Further, the prior art target structures have somewhat spherical orcurved pn-junctions which are self-formed during the evaporation of asuitable p-conductive material. Such curved pn-junctions exhibit a highfield strength at their thin edges which tends to destroy thepn-junctions and produce a short circuit within the target structure.

Yet further, the prior art structure has a very unfavorable resolutionpower in regard to a scanning electron beam. Apparently, this is due, atleast in part, to the circular form of the diodes utilized and to thelarge insulating distances between such diodes. In accordance with theprior art, the diameter of a scanning electron beam in an idealsituation is of a size encompassing five diodes. However, in the leastideal situation, the same size beam will only encompass four diodes(i.e., photo-electrical cells). Accordingly, the ratio of the targetsurface utilized in these two instances is about 5:4. Assuming a uniformillumination, this relation causes a brightness fluctuation of about 25percent and in practice, this tends to be very disturbing during theoperation of a camera tube (i.e., an electron tube that converts anoptical image into an electrical television signal).

Accordingly, it is an object of the invention to overcome the aforesaidprior art drawbacks and to provide a novel target structure for cameratubes having greatly improved properties and to provide methods ofproducing the same.

SUMMARY OF THE INVENTION In general, the invention provides a cameratube target structure comprised of a n-conductive signal plateexemplarily composed of Si on which a p-conductive layer is created orformed (i.e., as by diffusion of boron). An array of relatively narrowdeep grooves are provided so as to extend from the p-conductive layerinto the signal plate and intersect the p-conductive layer so as todefine an array of p-conductive regions having plane surfaces with endedges over-lying side walls of the grooves. The peripheral grooved wallsare coated with an insulator material, exemplarily composed of SiO whilethe island-like p-conductive re gions are coated with a conductivemetal, exemplarily composed of Au. The grooves and p-conductive regionsare configured to define distinct plane-like pnjmctions that providerelatively large individual diodes so that the effective target surfacearea for a scanning electron beam is materially increased and theresolution power likewise increased.

Additionally, the invention provides methods of producing such cameratube target structures wherein n-conductive signal plate is firstprovided with a suitable p-conductive material. Then a plurality ofindividually distinct metal islands or the like are provided on thep-conductive layer and the resultant structure is subjected to asuitable etch treatment whereby the areas between the metal islands areremoved to define an array of narrow deep grooves extending through thep-conductive layer and into the signal plate so as to distinctly cleaveor separate the pn-junction into a plurality of pnjmctions eachcorresponding to a discrete diode. The peripheral grooved walls are thencoated with an insulator layer. In certain embodiments, the metalislands are galvanically reinforced with a suitable etch-resistant metalprior to the etch treatment. Additionally, the metal islands areconfigured into a regular or irregular configuration dependent upon themeans utilized to provide the same.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged partial view ofa prior art target structure; 7

FIG. 2 is a partial plane view of the target surface of the structureillustrated at FIG. 1;

FIG. 3 is an enlarged partial elevational view of a target structure inaccordance with an embodiment of the invention; and

FIG. 4 is a partial plane view of the target surface of the structureillustrated at FIG. 3.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS Description of theoperational principles of a camera tube target structure is well knownto workers in the art and description thereof can be ascertained from anumber of sources, including the aforesaid U. S. Pat. No. 3,403,284(which is incorporated herein by reference).

As shown in FIG. 1, prior art target structures corn-. prise an-conductive Si-plate 1 having an insulator layer 2 (composed of SiOalong a surface thereof (i.e., the target surface). An array of recesses3 extends below the coated surface and into the body of the signalplate 1. A p-conductive material 4 is diffused into the recesses so asto form a plurality of diodes 3a. Such prior art target structures areformed by the Planar technique. Accordingly, the pn-junctions- 5 thatare formed thereby are somewhat sperically shaped. These curved surfacesor junctions cause a high fieldstrength to occur at their thin edges,i.e., the peripheral edges defining the recesses. Such high fieldstrengths tend to destroy the pn-junctions during operation, i.e.,causing a break-through to occur. In addition, a capacitance forms atthe ring-shaped edges of the recesses 3 that cause an additionalinterference of picture storage during operation. This capacitance formsbetween the upper and lower surfaces of the ring-shaped edgesof theinsulator layer 2 at the mouths of the recesses 3; these edges of layer2 limit the pn-junctions. The formed capacitance produces an unfavorabletime constant with the resistance of an equalization layer 6,

ally limits the bake-out temperature of a camera tube having suchatarget structure construction and thereby limits the life of the cameratube.

Additionally, as seen at FIG.v 2, prior art target structures have anunfavorable resolution power, primarily due to the circular shape of thediodes 3a. In accordance with the prior art, the diameter of an electronbeam 1 1, shown in dotted line, is equal to about double the gridconstant of the raster division. Accordingly, in an ideal alignment (asshown at the left hand side of FIG. 2) five diodes are encompassed bythe beam 11 while in the least favorable alignment (as shown at theright hand side of FIG. 2) only four diodes are encompassed by the beamll. Disregarding the fact that the target surface area covered by suchdiodes 3a is unfavorable, the relationship between such alignment,assuming a uniform illumination, results in a brightness fluctuation ofabout 25 percent. This degree of fluctuation is generally unacceptable.

While a possible solution to such a problem might appear present inmerely enlarging the dimension of the target and the diameter of theelectron beam, it

must be noted that such modification results in diminishing theresolution power. On the other hand, brightness fluctuation wouldincrease catastrophically if only the electron-beam diameter wasdecreased, since then a modulation of up to percent would result.

FIG. 3 illustrates a somewhat schematic cross-sectional view of aportion of a target structure 10 constructed-and operated in accordancewith the princi ples of the invention.

The vidicon target structures of the invention are produced inaccordance with the Mesa-technique. A light-permeable n-conductivesignal plate lq, exemplarily composed of Si, is suitably provided with alayer 18 of a suitable p-conductive' material, such as by evaporation orthe like. In this manner, a distinct planelike pn-junction 15 is formedalong the target surface of the structure 10. Thereafter, a conductivemetal, such as a noble metal, for example gold, platinum or the like, issuitably provided as an array of discrete individual metal island 17.Metal island 17 can be provided by the use of a mesh-wire mask or grid(not shown) applied to the target surface prior to the evaporation of aconductive metal, with subsequent removal of such mask; or islands 17can be provided by a technique similar to the one utilized for theproduction of rasters in iconoscopes whereby a conductive metal, such assilver, is provided as a continuouscoating and then disrupted or tom-upinto a'large number of irregularly shaped metal islands forming a mosaicon the target surface.

In certain instances, such as when a non-etch resistant metal isutilized, i.e., silver, to provide the metal islands, it is desirable toprovide (galvanically) a protective coating (not shown) of anetch-resistant material, such as gold, platinum or the like onto suchislands prior to etching. This latter means of providing distinct metalislands yields a target surface area having a very high percentagethereof covered by diodes 17a and thus allows the production of a targetstructure having a very high resolution power with a very low brightnessfluctuation.

After the array of metal islands 17 has been provided, a matchingarray'of grooves 19 are provided between the various distinct island 17so as to define an array of discrete diodes 17a. The grooves 19 arerelatively narrow and deep. The grooves 19 have somewhat concaveperipheral side walls so that the distance between opposite walls issufficient to insulate the individual pn-junctions (or diodes). Thedepth of the grooves 19 extends sufficiently through p-conductive layer18 and intothe body of signal plate 1a to insure that a completeseparation or cleavage of the plane pnjunction 15 occurs at the grooveareas and provides an array of individual diodes 17a. The grooves 19 areprovided, for example, by an etching process utilizing, for

example, a mixture of hydrofluoric acid and nitric acid. As shown on thedrawings, the grooves 19 each have a relatively narrow mouth portionwith interconnecting concaved peripheral walls. The concave walls ofgrooves 19 are coated with a layer 12 of an insulator material, such asSiO formed by an anodic oxidation during a gas discharge.

In the embodiment wherein a mesh grid or mask is utilized to provide thediscrete individual metal islands,

the mask is preferably selected so as to have extremely thin wireshaving diameters of only a few microns (i.e., p.). In the preferredembodiment, the mask is so selected as to provide a linear distanceratio of about 1:5 to 1:10 of the groove axial dimension to the diodeaxial dimension. In other words, the groove areas define only about topercent of the target surface area while the diode areas define about 80to 90 percent of the target surface areas.

In a preferred embodiment, the mesh or mask structure is so selectedthat it provides metal islands having dimensions (i.e., a surface area),which as an average are only slightly smaller than the cross-sectiondimension or diameter of a scanning electron beam. In other words theelectron beam diameter is adjusted to barely encompass the surface areaof a metal island so as to provide a target structure having improvedthe solution power.

The insulator layers formed between the adjacent diode of the inventionare preferably produced by means of an anodic oxidation in a gasdischarge.

In the embodiment wherein a mask is utilized to define the discretemetal islands during the evaporation ofa conductive metal, generallyonly metal island coniigurations of a geometrically regular shape, i.e.,rectangular, hexagonal, etc., are provided. However, even in theirsimplest shape, i.e., rectangular, the target surface area coveredthereby yields a much better covering factor as compared to the circularshape provided in accordance with the Planar technique.

In the embodiment wherein the irregular mosaic metal islands areprovided, a coherent or continuous metal layer, such as silver, isapplied to the p-conductive layer without the use of any mask.Thereafter, this continuous metal layer is thermally split or torn openin a known manner, such as exemplified in the production of rasters foriconoscopes. This method produces metal islands having highly irregularshapes which provide an extremely high covering factor on the targetsurface area. However, a silver mosaic of such metal islands is notsufficiently etch or acid resistant to allow the production of thegrooves. Accordingly, such non-etch resistant islands are galvanicallycovered or strengthened by the use of an electrolytical known processwherein a suitable etch-resistant material, such as gold, platinum orthe like is applied as a protective layer on top of the silver islands.This method of providing metal islands on the target structure isextremely advantageous since it is relatively simple and economical, andprovides a highly increased resolution power due to the high coveringfactor of the metal islands and does not produce any interfering effector brightness fluctuations.

The target structures of the invention have extremely favorableproperties for the operation of camera tubes, for example a Si-diodevidicon tube. When compared to prior art target structures (i.e., FIGS.land 2) the target structure of the invention has a much greaterutilizable effective surface area, as best seen at FIG. 4. In accordancewith the invention, the electron bearn 11 (shown in dotted line) isselected to have a diameter of about 17.5 t so as to just encompass thesurface area of one of the metal islands 17. As can be visualized fromthe showing at FIG. 4, this relation insures that even maximummodulation is less than about IO percent.

The relatively fine groove axial dimension of openings 19 and therelatively large useful surface areas of the diodes 17a provide aresolution factor of two on equal grid constants as compared with therelatively small round useful surface areas of diode members 3a formedwith the Planar methods.

In accordance with the principle of the invention, one can easily complywith the normal or common television standard of 625 by utilizing anumber of one million cells (i.e., diodes) and a grid constant of about12 t. With the target structure of the invention the maintenance of aparticular electron beam diameter is much less critical since it is onlybe decreasing the beam diameter to about 3 ,u that a modulation of up to100 percent would result.

()ne of the principle advantages of utilizing the described mesa-likeconfigurations of islands 17 is that the formation of interferingcapacitance is avoided. The configuration provided by the inventionallows a scanning electron beam to primarily strike metal surfaces (ofislands 17) and thereby directly discharge the individual diodes 17a. Inthose instances where an elec tron passes through the mouth of a groove19 and strikes thesurface of insulator layer 12, the portion of thecurrent influenced thereby is very small in spite of the strong negativepotential present at such insulator layers. Such negative potentialcannot cause an interfering effect at the target surface to a slowscanning electron impinging thereagainst, since it is confined withinthe groove. Storage capacitances such as that occurring with the priorart structures at the ring-like edges of the diode recesses are avoidedwith the target structure of the invention.

Another advantage of the invention resides in the excellent protectionfor the individual mesa-like diodes 17a from all types of pollution bythe protective metal islands. Of course, this protection is establishedrelatively early during the production of the diodes and is thencontinuously present. The mesa-type diode configuration also insuresthat relatively large distances are provided therebetween with regard toa mutual influencing of respective space-charge clouds. Accordingly, therastering is adjustable, if desired, to such a fine size that it is muchsmaller than the finest rastering available with prior art Planartechniques.

A practical means of achieving such fine rastering is by the utilizationof a fine grid mask in the method embodiment utilizing the same. In thatinstance, the suitably sized grid mask is applied directly to'the targetsurface of the Si-plate (i.e., signal plate) in a known manner and thenremoved after the evaporation of a suitably conductive metal.

Another practical means of achieving even finer rastering is by theapplication of a thermally torn or split metal mosaic in accordance withthe method embodiment utilizing the same. In this instance, the formedp-conductive layer (i.e., as by diffusion of B or the like) and which issuitably doped, defines the surface of the Si-plate and is covered orcoated with a continuous layer of a suitable metal, such as silver,without the use of any mask or the like. The silver coating is thenthermally disrupted into a very large number of irregularly shapedcompletely separated metal islands so as to provide a metal mosaic onthe target surface. The destruction of the silver coating is similar tothat utilizaed in the production of raster for iconoscopes.

In certain instances, the metal applied for the mosaic production asdescribed is insufficiently etch-resistant and must be provided with asuitable etch-resistant protective coating. Such protective coating isreadily provided by galvanic application of a suitable metal, such asgold, platinum or some similar material. The metal mosaic thus producedcovers a very large area of the target surface (i.e., has a largecovering factor) and thus provided a very high resolution power withvery low brightness fluctuation.

The semiconductor-type diodes of the invention are composed of materialsboth from the Ill-V groups and the ll-Vl and are particularly useful asconnecting semiconductors.

The invention is very versatile and finds a wide area of usage, over andbeyond the described embodiments.

In general, some of the principal advantages of the target structure ofthe invention are: that plane-like pnjunctions are provided, whichlessens the danger of a break-through or a short circuit of thepn-junctions at points or areas of curvature; that no additionalsemiconductor layers (i.e., equalization layers) are required for theelimination of interfering loads or potentials; that improved spacing ofthe pn-junctions of the individual diodes is achieved; that extremelyfine rasters areutilizable; and that improved resolution power isachieved by the improved covering factor and the smaller electron-beamdiameter which is utilizable, in comparison with the prior artstructure.

in summation, the invention provides a storing light sensitive targetstructure for camera tubes, such as semiconductor-type diode vidicontubes having a plurality of layers each having different conductivity soas to define individual diodes having pn-junction's which are insulatedfrom one another arranged on a lightpermeable n-conductive semiconductorsignal plate wherein the pn-junctions are of a plane configurationsufficiently separated from one another by relatively deep grooves todefine discrete large surface area diodes, which are covered with acorresponding sized insulated metal island.

In termsof construction, the target structure of the invention comprisesa n-conductive signal plate having a plane target surface with an arrayof grooves extending below such surface. The target surface areasbetween the grooves have a p-conductive layer thereon so that planepn-junctions form and define an array of individual diodes. Thep-conductive layer extends slightly beyond the peripheral side walls ofeach of the grooves and this layer is coated with a conductive metal sothat the metal coating extends somewhat further beyond the peripheralside walls of each of the grooves but does not close or block the mouthsthereof. The side wall portions of the signal plate and the p-conductive'Iayer exposed within the grooves have a coating of insulator materialthereon. The total effective surface area of the plurality of individualdiodes is equal to about 80 to 90 percent of the target surface areawhile the surface area of the groove mouths only equals about l topercent of the target surface area.

In terms of production, the invention comprises forming a layer of ap-conductive material onto a plane target surface of a n-conductivesignal plate and then providing an array of distinct metal islands orthe like of a given surface area on such p-conductive layer. Thereaftera corresponding array of grooves are etched between the metal islandsfor a depth sufficient to provide plane pn-junctions and definemesa-like individual diodes having an effective surface areacorresponding to the area of the metal islands. The peripheral groovewalls are coated with an insulator layer. The diodes are of. regular orirregular size depending upon the configuration of the metal islandsprovided and provide a scanning beam an effective target surface that issubstantially larger than the. exposed insulator layer surface.

In somewhat different terms, the invention provides an electron beamstorage light-sensitive target structure for a camera tube comprised ofa n-conductive signal plate having a target surface. An array of groovesare provided on this surface spaced from one another and extend belowthe surface. An array of mesa-like pconductive regions are provided onthe target surface between each of the grooves so that each of themesalike regions define with the underlying target surface a planepn-junction. A layer of a conductive metal is provided on each of themesa-like regions and a layer of an insulator material is provided alongthe peripheral walls of the grooves. The invention also provides methodsof producing such target structures comprising forming a p-conductivelayer onto a target surface of a n-conductive signal plate so that aplane pn-junction is formed at the interface of the layer and the targetsurface; providing an array of discrete conductive islands on thepconductive layer of the target surface and etching grooves between thediscrete islands so as to define individual diodes on the targetsurface, and then coating the peripheral walls of the openings with aninsulator layer.

Various modifications and alterations can be. effected as desiredwithout departing from the scope and spirit of the novel concepts of thepresent invention.

[claim 1. An electron beam storage light-sensitive target tending belowsaid target surface, an array of p-conductive regions each having aplane surface on said target surface between each of said grooves, eachof said p-conductive regions defining with the underlying target surfacea plane pnjunction, a layer of a conductive metal on the plane surfaceof each of said p-conductive regions to define a plane uppermost targetsurface, and a layer of an insulator material on the peripheral walls ofsaid grooves.

2. An electron beam storage light-sensitive target structure as definedin claim 1 wherein the p-conductive regions have a larger surface areaexposed to a scanning electron beam than the similarly exposed surfacearea of the grooves.

3. An electron beam storage light-sensitive target structure as definedin claim 1 wherein the grooves have concave peripheral walls terminatingin a relatively narrow mouth portion and the p-conductive regions haveperipheral side edges which extend over said concave walls of thegrooves.

4. An electron beam storage light-sensitive target structure as definedin claim 1 wherein the ratio of an axial linear dimension of a groove toan axial linear dimension of a p-conductive region is in the range ofabout 1:5 to 1:10.

tive regions have a geometrically regular surface configuration.

7. An electron beam storage light-sensitive target structure as definedin claim 1 wherein the conductive metal layer has a geometricallyirregular surface configuration.

2. An electron beam storage light-sensitive target structure as definedin claim 1 wherein the p-conductive regions have a larger surface areaexposed to a scanning electron beam than the similarly exposed surfacearea of the grooves.
 3. An electron beam storage light-sensitive targetstructure as defined in claim 1 wherein the grooves have concaveperipheral walls terminating in a relatively narrow mouth portion andthe p-conductive regions have peripheral side edges which extend oversaid concave walls of the grooves.
 4. An electron beam storagelight-sensitive target struCture as defined in claim 1 wherein the ratioof an axial linear dimension of a groove to an axial linear dimension ofa p-conductive region is in the range of about 1:5 to 1:10.
 5. Anelectron beam storage light-sensitive target structure as defined inclaim 1 wherein each of the p-conductive regions have a surface area ofa size that is at least encompassed by the diameter of a scanningelectron beam.
 6. An electron beam storage light-sensitive targetstructure as defined in claim 1 wherein the p-conductive regions have ageometrically regular surface configuration.
 7. An electron beam storagelight-sensitive target structure as defined in claim 1 wherein theconductive metal layer has a geometrically irregular surfaceconfiguration.